Charged-particle-beam (“CPB”, e.g., electron beam or ion beam) microscopes are in routine use for observing and inspecting intricate and highly integrated semiconductor circuits and the like as formed on suitable substrates. Such CPB microscopes include scanning electron microscopes (SEMs) and “mapping electron microscopes.” Whereas an SEM performs illumination and imaging from one point to another point on a specimen, a mapping electron microscope performs illumination and imaging from one surface to another surface of the specimen. Much research and development has been directed in recent years to improving the CPB mapping projection-optical systems used in mapping electron microscopes.
The structure of a conventional mapping electron microscope is summarized below, with reference to FIG. 1. A primary electron beam (also termed an “irradiation electron beam”) PB is emitted by an electron gun 21. The primary electron beam PB passes through an irradiation lens system 22 and enters a Wien filter 25. The Wien filter 25 typically comprises a magnetic pole 26 and an electrical pole 27. The Wien filter 25 bends the trajectory of the primary electron beam PB. After passing through the Wien filter 25, the primary electron beam PB passes through an aligner 30 and through an objective lens system 24 so as to be incident on the surface of a specimen 23. The irradiation lens system 22, Wien filter 25, aligner 30, and objective lens system 24 collectively are termed the “irradiation-optical system” or “primary optical system.”
Impingement of the primary electron beam PB on the surface of the specimen 23 generates relatively high-energy electrons that are reflected from the surface of the specimen 23 and relatively low-energy secondary electrons that are emitted from the surface of the specimen 23. The secondary electrons are normally used for imaging. The secondary electrons (formed into an “observation electron beam” or “secondary electron beam” OB) return through the objective lens system 24 and the aligner 30 and re-enters the Wien filter 25. Rather than experiencing trajectory bending by the Wien filter 25, the observation electron beam OB passes straight through the Wien filter 25. The observation electron beam OB then passes through an imaging lens system 28 and enters a detector 29. Observations of the specimen 23 are based on information in the observation electron beam OB as detected by the detector 29. The objective lens system 24, aligner 30, Wien filter 25, and imaging lens system 28 collectively comprise a “mapping optical system” or “secondary optical system.”
The Wien filter 25 is an electromagnetic prism also termed an “E×B” (“E cross B”). By imposing Wien's condition on the primary electron beam PB, the Wien filter 25 imparts a desired deflection to the trajectory of the primary electron beam PB, while not deflecting the trajectory of the secondary electron beam OB. Upon passing through the Wien filter 25, the primary electron beam PB can have, e.g., a linear, rectangular, circular, or elliptical transverse (sectional) profile.
It is necessary to be able to adjust accurately various components of the CPB mapping projection-optical system (e.g., align the illumination field of the primary optical system with the observation field of the secondary optical system) before use in order to observe and inspect the surface of the specimen 23 accurately. To such end, it would be advantageous to be able to adjust (e.g., alignment with optical axis, aberration correction) independently the primary optical system, the secondary optical system, and the Wien filter 25 (e.g., by adjusting respective voltages (or currents) applied to components in the primary optical system, the secondary optical system, and the cathode lens, and by adjusting the electromagnetic field generated by the Wien filter 25). Conventional adjustment methods require excessive time and effort to perform.
It also would be advantageous to be able to determine positional coordinates of the specimen being observed or inspected using a CPB mapping microscope. According to one conventional scheme for making such a determination, an off-axis light-optical system (i.e., an optical system for light) is used in conjunction with the CPB-optical system. In such a scheme, the specimen is mounted on a stage provided with fiducial marks (e.g., a pattern of lines and spaces). Unfortunately, however, conventional practice has revealed much difficulty in detecting such marks using both a light-optical system and a CPB-optical system. Difficulty is also conventionally encountered in detecting fiducial marks configured as a grooved pattern (e.g., scribe lines), which readily can be detected using an optical microscope but not by a CPB-optical system.
In other words, marks that can be detected readily using light are usually not detectable using a charged particle beam. This makes it difficult to select a fiducial mark that is optimal for use with both a CPB-optical system and an off-axis light-optical system.
According to another conventional method for evaluating optical performance (e.g., resolution and aberration) of a CPB mapping microscope, an “evaluation chart” is placed at the position of the specimen 23 in FIG. 1. The evaluation chart is typically a pattern comprising ultra-fine features defined by deposition or microlithography. The evaluation chart is irradiated using the primary electron beam PB, and an image is produced from the observation beam OB generated therefrom.
Unfortunately, whenever optical performance is evaluated using an evaluation chart in such a manner, the optical axis of the irradiation-optical system and the optical axis of the mapping optical system must be adjusted simultaneously by making simultaneous adjustments to the Wien filter and the aligner. This requires that the evaluation chart be illuminated uniformly with the primary electron beam PB in order to check the optical performance of the mapping electron microscope. The Wien filter's condition is found while continuously changing the electromagnetic-pole induction parameters in the Wien filter 25 so that the trajectory of the secondary electron beam is not deflected. Changing the electromagnetic-pole induction parameters in such a manner causes a simultaneous change in the uniformity of illumination by the primary electron beam. Consequently, it is necessary to readjust the optical axis of the illumination optical system continually. In addition, whenever the secondary electron beam is deflected by the aligner and axially aligned with the objective lens system, the primary electron beam is simultaneously deflected, thereby changing the uniform illumination and making it necessary again to readjust the optical axis of the illumination optical system. Thus, such conventional evaluations of optical performance are extremely complex to perform.
The kinetic-energy distribution of electrons in the secondary electron beam emitted from the specimen is very sensitively affected by the type and shape of the specimen and the irradiation angle of the secondary electron beam. This instability of the kinetic-energy distribution of the secondary electron beam adds even more complexity to conventional evaluations of the optical performance of the mapping electron microscope, and makes it impossible to determine, e.g., the magnitude of chromatic aberration.